PROGRAM TURTLE;

CONST
  ON = TRUE;
  OFF = FALSE;
  ENDKEY = 79;
  UPARROW = 72;
  DNARROW = 80;
  LTARROW = 75;
  RTARROW = 77;

VAR
  X:INTEGER;
  Y:INTEGER;
  ANGLE : INTEGER;
  PENCOLOR : INTEGER;
  KEY : INTEGER;
  XSTART:INTEGER;
  YSTART:INTEGER;
  XEND:INTEGER;
  YEND:INTEGER;


PROCEDURE DELAY;
VAR N:INTEGER;
BEGIN
  FOR N := 1 TO 10 DO;
END;


PROCEDURE FINDMOVE(ANGLE:INTEGER;
                   VAR DX,
                       DY:INTEGER);
VAR
  N : INTEGER;
BEGIN
  N := ANGLE / 10;
  CASE N OF
   0: BEGIN DX := 0; DY := -100;   END;
   9: BEGIN DX := 100;   DY := 0; END;
   18:BEGIN DX := 0; DY := 100;  END;
   27:BEGIN DX := -100;   DY := 0; END;
  END;
END;

PROCEDURE FORWARD(DISTANCE:INTEGER);
VAR
  DX:INTEGER;
  DY:INTEGER;
BEGIN
  FINDMOVE(ANGLE,DX,DY);
  DX := DX*DISTANCE/100;
  DY := DY*DISTANCE/100;
  DRAW(X, Y, X+DX, Y+DY, PENCOLOR);
  X := X + DX;
  Y := Y + DY;
END;

PROCEDURE TURTLE(ON:BOOLEAN);
VAR
SAVEPC : INTEGER;
BEGIN
  SAVEPC := PENCOLOR;
  PENCOLOR := PENCOLOR + 128;
  IF ON THEN
    BEGIN
      XSTART := X;
      YSTART := Y;
      FORWARD(5);
      XEND := X;
      YEND := Y;
      X := XSTART;
      Y := YSTART;
    END ELSE
    BEGIN       {erase turtle}
      DRAW(XSTART,YSTART,XEND,YEND,PENCOLOR);
    END;
  PENCOLOR := SAVEPC;
END;


BEGIN {main program}
  SETMODE(4);
  PENCOLOR := 3;
  X := 160;
  Y := 100;
  ANGLE := 0;
  REPEAT
    REPEAT
      TURTLE(ON);
      DELAY;
      TURTLE(OFF);
      DELAY;
    UNTIL KEYPRESSED; {you can't step past this line, so  }
    KEY := ORD(INKEY);{ Insert a stop on this line}
    IF KEY MOD 256 = 0 THEN KEY := KEY/256 ELSE KEY := 0;
    IF KEY IN [UPARROW,DNARROW,LTARROW,RTARROW] THEN        {arrow keys}
     CASE KEY OF
       UPARROW: FORWARD(10);
       DNARROW: FORWARD(-10);
       LTARROW: IF ANGLE = 0 THEN ANGLE := 270 ELSE ANGLE := ANGLE - 90;
       RTARROW: IF ANGLE = 270 THEN ANGLE := 0 ELSE ANGLE := ANGLE + 90;
      END;
  UNTIL KEY = ENDKEY;
END.